1. Field of the Invention
This invention relates to a perpendicular magnetic recording medium, which is mountable in an external storage device for a computer or in other magnetic recording devices.
2. Description of the Related Art
Perpendicular magnetic recording methods are beginning to be commercialized as technology to achieve high densities in magnetic recording, in place of conventional longitudinal magnetic recording methods.
A perpendicular magnetic recording medium is generally formed by sequentially layering a soft magnetic backing layer, a nonmagnetic intermediate layer, a magnetic recording layer, and a protective layer. As the nonmagnetic intermediate layer, Ru or an alloy containing Ru is used. As the magnetic recording layer, materials having a so-called granular structure, which includes ferromagnetic crystal grains mainly comprising CoPt alloy having ferromagnetic properties and nonmagnetic grain boundaries mainly comprising an oxide and surrounding the ferromagnetic crystal grains, are widely used.
For example, a perpendicular recording medium, having a nonmagnetic intermediate layer of Ru and a magnetic recording layer of CoPtCr—O alloy with the so-called granular structure, is described in Oikawa et al., “High Performance CoPtCrO Single Layered Perpendicular Media with No Recording Demagnetization”, IEEE Transactions on Magnetics, Vol. 36, No. 5, p. 2393-2395, September 2000. Here, as the film thickness of the Ru, which is the nonmagnetic intermediate layer, is increased, the c-axis orientation of the CoPtCr—O alloy in the magnetic recording layer is improved, isolation of magnetic crystal grains is promoted, and as a result, excellent magnetic characteristics and electromagnetic transducing characteristics are obtained.
Further, in the Specification of U.S. Pat. No. 7,067,206, a perpendicular magnetic recording medium is disclosed having, in order, a soft magnetic backing layer, an orientation control layer comprising an alloy with a face-centered cubic (fcc) structure, a nonmagnetic intermediate layer of Ru having a hexagonal close-packed (hcp) structure, and a magnetic recording layer of a CoPtCr—SiO2 alloy having a granular structure. Here, by providing an orientation control layer, further control of the fine structure in the nonmagnetic intermediate layer and magnetic recording layer is possible, so that even more satisfactory electromagnetic transducing characteristics can be realized.
In Japanese Patent Application Laid-open No. 2004-288348, a perpendicular magnetic recording medium is disclosed in which an underlayer comprising soft magnetic Permalloy material and a soft magnetic Co layer or a soft magnetic Co-base alloy layer are provided between a soft magnetic backing layer and a nonmagnetic intermediate layer comprising Ru or Ru-base alloy. Here, by providing a soft magnetic Co layer or a soft magnetic Co-base alloy layer, the coercivity of the magnetic recording layer is increased, the squareness ratio of the medium is improved, and simultaneously the thickness of the Ru or Ru alloy film serving as the nonmagnetic intermediate layer can be reduced. As a result, media noise is decreased and the SNR (signal-to-noise ratio) is improved.
In order to further raise the recording density of a perpendicular magnetic recording medium, reduction of the thickness of the intermediate layer between the soft magnetic layer and the magnetic recording layer is extremely important; but a dilemma results in that there is also a need for the intermediate layer to be at least of a certain thickness in order to promote magnetic isolation of magnetic crystal grains in the magnetic recording layer and to improve crystal orientation. Further, if the intermediate film thickness is decreased in order to raise densities, there is a corresponding increase in noise arising in the soft magnetic layer, so that characteristics are degraded.